Many techniques are currently available for the detection of metals in the environment. The development and improvement of these techniques has become a major focal point of analytical science because of the growing need to detect very small amounts of metallic contaminants which adversely affect the environment. For example, mercury is regarded as a very toxic heavy metal, and its presence in soil and waterways represents a considerable health hazard. Government agencies throughout the world are thus increasing restrictions on the release of mercury to the environment. In some countries, a legislated limit of 2 parts-per-billion in drinking water has been enforced. Other potentially hazardous metals like lead and cadmium appear to be receiving the same scrutiny.
The most commonly used methods for detecting various metals are atomic absorption (AA), inductively coupled plasma atomic emission (ICP-AE), and mass spectroscopy (MS). Each of these methods is suitable for trace analysis of metals like mercury in a laboratory setting. However, they often require well-controlled experimental conditions, expensive instrumentation, and frequent maintenance and calibration. Moreover, these methods usually require lengthy sample preparation, especially when other interfering metals or impurities are present in the sample under investigation. For these reasons, the methods mentioned above are not particularly well-suited for rapid analysis in the field.
Other methods which are sometimes suitable for metal detection and analysis in the field include X-ray fluorescence (XRF), colorimetry, and ion-selective electrode (ISE). Special mention is made of XRF, which is used in the field because of its suitability for simultaneously detecting many metals without substantial sample preparation. However, the detection limits for this method (about 30-100 ppm for mercury in soil samples) are not low enough for accurately determining very low levels of metals like mercury. Moreover, XRF is very dependent on the nature of the environmental sample. For example, if one is running a mercury analysis on both a soil sample and a plastic sample, a separate calibration curve must be prepared for each.
Colorimetric techniques can be complicated and time-consuming. Also, such techniques are often very specific, e.g., selective to only one type of mercury complex.
ISE is a useful detection tool in some instances, but it is usually limited to the detection of inorganic ions. Furthermore, measurement is heavily dependent upon the specific composition of a sample substrate, since the potentiometric response varies not only with the particular metal being investigated, e.g., mercury, but also with varying concentrations of halides and sulfides which may constitute part of the substrate.
Because of its high sensitivity, Anodic Stripping Voltammetry (ASV) is a very popular laboratory tool for the detection of heavy metals. However, the method is not typically used for mercury because mercury is the preferred working electrode in ASV. The limitation of this method in field analysis, especially for solid wastes, is mainly due to lack of fast extraction of metals to an aqueous solution and the deactivation of working electrode.